A. Field of the Invention
The present invention relates generally to electromagnetic interference (EMI) shielding techniques and more particularly to a method of forming EMI shielded enclosures, and an apparatus for shielding an enclosed volume by insulating the enclosed volume from interfering background electromagnetic waves and from the conductive effects of moisture.
B. Description of the Prior Art
EMI shielded enclosures are used for many purposes including minimizing the susceptibility of data processing equipment to external surveillance and for isolating sensitive equipment from interference due to ambient electromagnetic radiation and other outside activity. For example, in magnetic resonance imaging (MRI) applications, EMI must be minimized to prevent artifacts, or pictorial flaws in the final image. Also, the superconducting magnets, an integral part of MRI, must be isolated from outside magnetic influences such as automobile traffic, elevators, or other activities involving movement of large ferrous objects that can distort the homogeneity of the magnetic field. Ideal EMI shields should also incorporate a seamless construction, particularly in the floor, whereby mechanical seams, found in typical enclosures, are eliminated so as to minimize ground-level corrosion and enhance load bearing characteristics.
A commonly used type of shielded enclosure is a room totally enveloped or surrounded by a layer of electrically conductive material, such as a metal plate, a metal sheet or a metal screen. One example of an EMI shielded enclosure is disclosed in U.S. Pat. No. 4,806,703, issued Feb. 21, 1989.
One EMI shielding procedure involves securing plates of metal shielding material, such as steel, to the floor, walls and ceiling of an enclosure (or containment room), that houses the magnetic resonance imaging device. For example, U.S. Pat. No. 4,651,099, discloses first covering a concrete-slab floor with water-impermeable plastic sheeting, and then placing asbestos board over the plastic sheeting. Finally, steel plating is placed over the asbestos flooring. The steel plating then can be covered with a decorative tile or other type flooring. Another EMI shielding procedure is disclosed by U.S. Pat. No. 4,755,630, including adhesively securing a plastic sheeting to the floor, then installing multiple layers of plywood, layed perpendicular to each other, onto the plastic sheeting. The plastic sheeting acts as a moisture barrier between the floor and the plywood. Aluminum sheets then are secured, such as with screws, onto the plywood subflooring.
Such shielding techniques are costly and labor intensive in that they involve elaborate preparation prior to the attachment of the metallic sheeting. These techniques also allow EM interference to develop due to the oxidation which often occurs between the nails, screws or similar mechanical fasteners and the EM shielding material whereby the fastener is no longer firmly grounded to the shielding material but instead acts as an antenna to receive any EM interference. While known methods and devices reduce EM interference, a need exists for an improved EMI shielded enclosure capable of, for example, essentially eliminating surreptitious monitoring of data processing activities, eliminating secondary ground paths via the floor which interferes with the troubleshooting of equipment, and also minimizing or eliminating EM interference that occurs due to the electrically-conductive effects of moisture between the metallic RF shielding material and a support substrate. Further, by eliminating all electrical contact between the EMI shielded enclosure and electrically grounded conductive members of the supporting parent structure, a single point ground system will be achieved which eliminates multiple ground loops and the associated stray currents flowing in the shielded enclosure surfaces which deteriorate shielding effectiveness.